Glycosylation of immunoglobulins has been shown to have significant effects on their effector functions, structural stability, and rate of secretion from antibody-producing cells (Leatherbarrow et al., Mol. Immunol. 22:407 (1985)). The carbohydrate groups responsible for these properties are generally attached to the constant (C) regions of the antibodies. For example, glycosylation of IgG at asparagine 297 in the CH2 domain is required for full capacity of IgG to activate the classical pathway of complement-dependent cytolysis (Tao and Morrison, J. Immunol. 143:2595 (1989)). Glycosylation of IgM at asparagine 402 in the CH3 domain is necessary for proper assembly and cytolytic activity of the antibody (Muraoka and Shulman, J. Immunol. 142:695 (1989)). Removal of glycosylation sites as positions 162 and 419 in the CH1 and CH3 domain of an IgA antibody led to intracellular degradation and at least 90% inhibition of secretion (Taylor and Wall, Mol. Cell. Biol. 8:4197 (1988)).
Glycosylation of immunoglobulins in the variable (V) region has also been observed. Sox and Hood, Proc. Natl. Acad. Sci. USA 66:975 (1970), reported that about 20% of human antibodies are glycosylated in the V region. Glycosylation of the V domain is believed to arise from fortuitous occurrences of the N-linked glycosylation signal Asn-Xaa-Ser/Thr in the V region sequence and has not been recognized in the art as playing an important role in immunoglobulin function.
It has been reported that glycosylation at CDR2 of the heavy chain, in the antigen binding site, of a murine antibody specific for α-(1-6)dextran increases its affinity for dextran (Wallick et al., J. Exp. Med. 168:1099 (1988) and Wright et al., EMBO J. 10:2717 (1991)).
M195 is a murine IgG2a monoclonal antibody that binds CD33 antigen and has therapeutic potential for the treatment of myloid leukemia (Tanimoto et al., Leukemia 3:339 (1989) and Scheinberg et al., Leukemia 3:440 (1989)). M195 binds to early myeloid progenitor cells, some monocytes, and the cells of most myeloid leukemias, but not to the earliest hematopoietic stem cells.
The efficient cellular binding and internalization of M195 has allowed use of the radiolabeled antibody in clinical trials for acute myelogenous leukemia (AML) (Scheinberg et al., J. Clin. Oncol. 9:478 (1991)). The murine M195 antibody, however, does not kill leukemic cells by complement-dependent cytotoxicity with human complement, or by antibody-dependent cellular cytotoxicity with human effector cells. The human anti-mouse antibody (HAMA) response may also preclude long term use of the murine antibody in patients. To increase the effector function and reduce the immunogenicity of the M195 antibody in human patients, chimeric and humanized versions of the antibody have been constructed (Co et al., J. Immunol. 148: 1149, (1992)). The chimeric antibody combines the murine M195 V region with a human C region, while the humanized antibody combines the complementarity determining regions (CDRS) of murine M195 with a human antibody V region framework and C region (Co et al., op.cit.). The construction and characterization of chimeric and humanized M195 antibodies of the human IgG1 isotype is described (Co et al., op.cit.).
While the production of so called “chimeric antibodies” (e.g., mouse variable regions joined to human constant regions) has proven somewhat successful in reducing the HAMA response, a significant immunogenicity problem remains. Moreover, efforts to immortalize human B-cells or generate human hybridomas capable of producing human immunoglobulins against a desired antigen have been generally unsuccessful, particularly with many important human antigens. Most recently, recombinant DNA technology has been utilized to produce immunoglobulins which have human framework regions combined with complementarity determining regions (CDR's) from a donor mouse or rat immunoglobulin (see, e.g., EPO Publication No. 0239400). These new proteins are called “reshaped” or “humanized” immunoglobulins and the process by which the donor immunoglobulin is converted into a human-like immunoglobulin by combining its CDR's with a human framework is called “humanization”. Humanized antibodies are important because they bind to the same antigen as the original antibodies, but are less immunogenic when injected into humans.
However, a major problem with humanization procedures has been a loss of affinity for the antigen (Jones et al., Nature, 321, 522-525 (1986)), in some instances as much as 10-fold or more, especially when the antigen is a protein (Verhoeyen et al., Science, 239, 1534-1536 (1988)). Loss of any affinity is, of course, highly undesirable. At the least, it means that more of the humanized antibody will have to be injected into the patient, at higher cost and greater risk of adverse effects. Even more critically, an antibody with reduced affinity may have poorer biological functions, such as complement lysis, antibody-dependent cellular cytotoxicity, or virus neutralization. For example, the loss of affinity in the partially humanized antibody HuVHCAMP may have caused it to lose all ability to mediate complement lysis (see, Riechmann et al., Nature, 332, 323-327 (1988); Table 1).
Therefore, there exists a need in the art for immunoglobulins that have an altered affinity for antigen, particularly an increased affinity and/or increased specificity for an antigen, and, desirably, potentially lower immunogenicity and improved effector function conferred by naturally-occurring constant region glycosylation. For example, an immunoglobulin having one or more human constant region effector functions and an improved binding affinity and/or specificity characteristic of the M195 antibody variable region may eliminate the need for radiolabeling and allow repeated does in therapeutic trails. Additionally, there is a need in the art for methods that produce immunoglobulins which have improved binding affinity and/or specificity for an antigen, but which do not have significantly increased immunogenicity. Thus, there exists a need in the art for methods to increase the efficacy and reduce the required doses of immunoglobulins of therapeutic importance, and immunoglobulins produced by such methods.